The present invention relates to devices which control radiant heat flow and has particular application as a thermal shield in gas turbine engines where it is used to protect the engine casing from the detrimental effects of hot gases.
Control of the radial propagation of heat by radiation and convection from an axially flowing hot gas stream to surrounding parts poses a significant problem to gas turbine engine designers. Excessive thermal cycling of these surrounding parts creates clearance problems due to material growth and shrinkage. Special materials must be used for these parts that are capable of withstanding the extreme high temperatures and thermal stresses encountered. These factors coupled with the vibrational stresses of engine operation can lead to premature fatigue failure of the parts, as well as their mounting provisions. As the temperatures of the (main) hot gas stream are increased to improve engine operating efficiencies, these problems are exacerbated.
One approach to controlling or limiting radial heat propagation is to utilize pressurized cooling air to limit the heat rise in the surrounding parts by impingement and convection cooling. Pressurized cooling air is also utilized to thwart radial leakage flow of hot gases through clearance gaps between those components defining the outer bounds of the main gas stream. Since pressurized cooling air is tapped from the compressor output leading to the combustor, engine performance and efficiency suffer as the amount of air utilized for cooling is increased. Thus, it is important to use cooling air efficiently and then only where absolutely necessary.
Another approach is to utilize thermal shielding to limit radial heat propagation. It will be appreciated that effective use of thermal shielding can reduce the amount of air cooling required or even eliminate the need for air cooling. One location where thermal shields are used to reduce radial heat transfer by radiation and convection is between the engine case and the vane tip platforms or outer bands of the first stage low pressure turbine nozzle. These thermal shields have taken the form of blankets of fibrous insulating material retained between inner and outer sheet metal liners. It has been found that these shields offer little structural resistance to pounding levels of oscillatory vibration, high temperatures due to hot gas streams, intense localized convection heat transfer, and abrasion damage from adjacent components, such as the casting upstanding from the outer nozzle bands. The combination of these destructive mechanisms degrades the insulative quality of the shields and eventually results in the complete disintegration thereof.